Quick reference for network engineers. Find subnet masks, host counts, and typical use cases for every CIDR prefix.
CIDR Reference Table
Every IPv4 prefix from /0 to /32 with the subnet mask, host counts, and where each size typically gets used — the table worth having open in a tab during any network design session, firewall rule review, or CCNA revision cram.
What the slash actually means
CIDR replaced the old class-based A/B/C system in 1993 for a good reason: classful addressing was wasteful, handing out whole Class B blocks (65,534 hosts) to organisations that needed a few hundred addresses. The number after the slash is how many bits of the 32-bit address are the network portion — everything left over is host space. It's just a more efficient way to slice up address space to match what you actually need.
The prefixes you'll actually use
- /8 — 16,777,216 hosts — historically Class A territory, enterprise/ISP scale
- /16 — 65,536 hosts — large corporate networks, or a whole cloud VPC
- /24 — 254 usable hosts — the standard office LAN segment, the one everyone knows by heart
- /25 — 126 usable — splitting a /24 in half
- /28 — 14 usable — small server segments
- /30 — 2 usable — the classic point-to-point WAN link size
- /31 — 2 addresses, no broadcast address at all, defined specifically for point-to-point links under RFC 3021
- /32 — a single host — loopbacks and static host routes
In cloud networking specifically
AWS's own guidance is a /16 for the VPC and /24 for individual subnets within it — and the important detail people miss is that a VPC's CIDR block can't be changed after creation without recreating it entirely. Plan the address space with room to grow before you provision anything, because "just add another subnet later" only works if you actually left the address space to do it.